DIATOM OOZE 



179 



tic, where it covers some relatively small areas, in depths of 400 

 to 1500 fathoms. 



(3) Radiolarian Ooze. — The organisms which we have so far 

 considered secrete only shells or tests of CaC0 3 , but this is not the 

 only substance which is very extensively extracted from sea-water 

 by living beings. Silica is also dissolved in sea-water, and various 

 organisms construct their tests of that substance. The Radiolaria 

 are, like the Foraminifera, a group of microscopic, unicellular 

 animals, which secrete siliceous tests of the most exquisite delicacy 

 and beauty ; they live both at the surface and at the bottom of the 



FIG. 74. — Pteropod ooze. X 4. (Agassiz after Murray and Renard.) 



sea. Radiolarian tests may be detected in all sorts of marine 

 deposits of both deep and shallow water, but it is only in very pro- 

 found depths that they occur in quantity sufficient to give character 

 to the deposit. When 20% or more of a bottom deposit consists 

 of radiolarian tests, it is called a radiolarian ooze, but clay and 

 volcanic minerals make up most of the materials. This ooze has 

 been found only in the Pacific and Indian oceans, where, it is esti- 

 mated, it covers 2,290,000 square miles of the bottom, at depths 

 of 2350 to 4475 fathoms. 



(4) Diatom Ooze. — In our study of fresh-water deposits we 

 learned that the siliceous cases of the microscopic plants known as 



